This invention relates to a method of depositing electroluminescent devices and more particularly to a method of forming an aluminum-lithium alloy cathode layer in such devices for improving the drive voltage and efficiency of these devices.
Organic light emitting devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated.
The most recent discoveries in the art of organic light emitting device construction have resulted in devices having the organic EL medium consisting of extremely thin layers ( less than 1.0 micrometer in combined thickness) separating the anode and cathode. The thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. In a basic two-layer organic light emitting device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. Examples are provided by U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,885,211; 4,950,950; 5,047,687; 5,059,861; 5,061,569; 5,073,446; 5,141,671; 5,150,006; and 5,151,629.
The simple structure can be modified to a three-layer structure, in which an additional luminescent layer is introduced between the hole and electron-transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the luminescent or recombination layer can be chosen to have a desirable EL color as well as high luminance efficiency. Likewise, the electron and hole transport layers can be optimized primarily for the carrier transport property.
The organic light emitting devices can be view as a diode, which is forward biased when the anode is at a higher potential than the cathode. The anode and cathode of the organic light emitting device can each take any convenient conventional form, such as any of the various forms disclosed by Tang et al. U.S. Pat. No. 4,885,211. Operating voltage can be substantially reduced when using a low work function cathode and a high work function anode. The preferred cathodes are those constructed of a combination of a metal having a work function less than 4.0 eV and one other metal, preferably a metal having a work function greater than 4.0 eV. The Mg:Ag of Tang et al. U.S. Pat. No. 4,885,211 constitute one preferred cathode construction. The Al:Mg cathodes of Van Slyke et al. U.S. Pat. No. 5,059,862 are another preferred cathode construction.
Organic EL cells using low work function alloys of Alxe2x80x94Li as a cathode were reported by Wakimoto et al. (IEEE Transactions on Electron Devices, vol. 44, 1245 [1997]). These cells performed well in both EL emitting characteristics and the durability. However, Wakimoto et al. reported that it was difficult to maintain the optimal Li concentration in the films. Alxe2x80x94Li alloy was deposited by using conventional thermal evaporation. Therefore, the EL cells with Alxe2x80x94Li cathode having optimal concentration did not reproduce well. For the fabrication of organic light emitting devices, it is important that the thin film preparation method is reproducible for large-scale manufacturing.
To eliminate the irreproducibility of the organic light emitting devices, Wakimoto used lithium oxide/aluminum metal bi-layer cathode. The use of a LiF/Al bilayer to enhance electron injection in organic light emitting devices has also been disclosed by Hung et al. in U.S. Pat. No. 5,624,604. However, this is a two-step process and involves the use of two evaporation sources, one for LiF and other for Al. This involves significant time loss during manufacturing. If two sources are located in a single evaporation chamber, then it requires the first LiF source to be turned ON, bring it to the evaporation temperature and then deposit 0.5 nm LiF layer. Before depositing the second 200 nm Al metal layer, either the substrates having the thermal sensitive organic light emitting layers should be shielded against the radiation heat from the second Al source, or it should be transported to the other buffer chamber before the second source comes up to high temperature and is ready to reach the required Al deposition rate. If two separate chambers are used for deposition of LiF and Al layers separately, this results in significant time loss for transporting substrate from one chamber to another chamber, in addition to the cost of the second chamber.
It is an object of the present invention to provide an improved method of making organic light emitting devices, which eliminates the above non-reproducibility of the Alxe2x80x94Li cathode and provide high quality organic light emitting devices.
This object is achieved in a method for making an electrode over a light emissive layer in an organic light emitting device, comprising the steps of:
a) providing the organic light emitting device into a vacuum chamber having a receptacle for vaporizing material, means for heating the receptacle to evaporate material placed in the receptacle for deposition onto the light emissive layer to form the electrode, and a shutter, which when open, permits evaporated material from the heated receptacle, to deposit onto the light emissive layer; and
b) selectively feeding an elongated member made of material to be evaporated into the heated receptacle when the electrode is to be formed and removing such material from such heated receptacle when such electrode is not to be formed.
The following include advantages of the present invention:
1) Use of Alxe2x80x94Li alloy as a cathode decreases the organic light emitting drive voltage as compared to Mgxe2x80x94Ag alloy.
2) The cathode deposition is a single-step evaporation process and uses a single evaporation source.
3) The present invention requires less deposition time and thus short cycle time. This can also reduce the possible thermal damage to the sensitive organic light emitting organic layers.
4) The present invention improves the production throughput for low cost manufacturing.
5) Flash evaporation deposits repeatable alloy composition for each deposition, thus minimizing variability from device to device.
6) A spool containing a thin wire up to 200 feet long can be used for preparing numerous devices. This eliminates the opening of the deposition vacuum chambers to atmospheric pressure and reduction of the down time.
7) The present invention increases the production efficiency.
8) Eliminates spitting of the alloy and the quality organic light emitting devices are prepared.
9) Reduces the materials usage.
10) Cathode deposition process has superior stability and control of the deposition flux, and very clean operation conditions.
11) Produces organic light emitting devices with Alxe2x80x94Li alloy cathodes with good reproducibility.